Method of centrifugally casting thin edged corneal contact lenses

ABSTRACT

A CORNEAL CONTACT LENS IS MADE BY POLYMERIZING A CATALYZED MIXTURE OF ACRYLATES AND/OR METHACRYLATES OF POLYHYDRIC ALCOHOLS WITH A WATER MISCIBLE SOLVENT IN A MOLD CAVITY HAVING A SPHERICALLY CONCAVE BOTTOM SURFACE AND BEING ROTATED ABOUT AN AXIS PERPENDICUAR TO THAT SURFACE AT A SPEED TO PRODUCE AN EXPOSED CONCAVE SURFACE OF THE   POLYMERIZATION MIXTURE, WHICH IS SUFFICIENTLY DIFFERENT FROM A PARABOLOID SHAPE BECAUSE OF SURFACE TENSION EFFECTS TO HAVE A CENTER EFFECTIVE AS A CORRECTIVE LENS.

'May 2, 1972 o. wicHTEQLE MELHOD OF' CENTRIFUGALLY CASTING THIN EDGILDCORNEAL CONTACT LENSES Original Filed Oct. 24, 1963 INVENTOR May 2, 1972o. wlcHTl-:RLE 3,650,545 METHOD OF CENTRIFUGALLY CASTING THIN EDGEDCORNEAL CONTACT LENSES Original Filed Oct. 24, 1963 3 Sheet-Sheet 2 20INVENTOR Uffa MCA ere May 2, 1972 o. WICHTERLE 3,660,545

METHOD OF CENTRIF'UGALLY CASTING THIN EDGED CORNEAL CONTACT LENSESOriginal Filed Oct. 24, 1963 3 Sheet-Sheet 5 IN VENTOR.

Uffa M'C fere United States Patent O Int. Cl. B29d 11/00 U.S. Cl. 264-18 Claims ABSTRACT OF THE DISCLOSURE A corneal contact lens is made bypolymerizing a catalyzed mixture of acrylates and/ or methacrylates ofpolyhydric alcohols `with a water miscible solvent in a mold cavityhaving a spherically concave bottom surface and being rotated about anaxis perpendicular to that surface at a speed to produce an exposedconcave surface of the polymerization mixture, which is sufficientlydifferent from a paraboloid shape because of surface tension effects tohave a center effective as a corrective lens.

This application is a continuation of my copending application Ser. No.810,072, tiled Jan. 31, 1969, now abandoned, itself a division of mycopending application Ser. No. 318,627, filed Oct. 24, 1963 nowabandoned, itself a continuation-in-part of my application Ser. No.229,727 filed Oct. 10, 1962, and now abandoned.

This invention relates to a method of preparing corneal contact lensesand more particularly contact lenses essentially consisting of softhydrogels of organic polymers.

Contact lenses are usually made of glass or of substantially rigidplastics. It was disclosed in U.S. Pat. No. 2,976,576 that certainsparingly cross-linked polymeric hydrogels such as those of acrylatesand methacrylates of polyhydric alcohols have optical and physiologicalproperties which make them eminently suitable lfor use as contactlenses. The hydrogels are soft so that they cannot mechanically injurethe eye, and they may be made isotonic with body iiuids so that theymaintain a desired degree of swelling when in contact with the eye.

Some difficulties were encountered heretofore in the manufacture ofcontact lenses from soft polymeric hydrogels. The polymerized materialis too soft when in the swollen state to permit surface finishing, andtoo brittle when dried. It is therefore necessary that contact lenses bemade from the hydrogels by polymerizing a suitable starting material ina mold whose surface configuration determines the lens surface.

In order to produce a useful lens, it was considered necessaryheretofore to retain the polymerization mixture between two suitablycurved mold surfaces. Shrinkage inevitably occurs during polymerization.If a completely closed mold were employed, the shrinking polymerizationmixture would pull away from at least one mold surface, and control overits surface shape would be ICC lost. In a known method for makingcontact lenses from the hydrogels referred to, a polymerization mixtureis therefore held between a concave mold member and a convex mold memberwhich are spaced from each other by an annular gap so that the moldinginitially formed has top and bottom surfaces respectively curved toconform to the mold members, and a rough annular edge connecting the topand bottom surfaces. The irregular configuration of the edge is causedby the shrinkage of the polymerization mixture.

The edge portion is removed by cutting, and the nished known lens has acut edge of appreciable thickness which tends to catch the eyelid duringmovement of the latter when the contact lens is used, whereby the lensmay be displaced from its normal position of alignment with the pupil ofthe eye. It would be desirable to avoid the possibility of suchdisplacement by having a feather edge on the lens. The edge should beshaped in such a manner that tangents, drawn on the outer and innerfaces of the lens in a common axial plane, form as small an angle as ispossible. Ideally, the two main surfaces of the lens should have acommon tangent at the edge.

Lens made of rigid material, such as glass or polymethylmethacrylate,may be ground to this edge conguration, but a sharp edge is notpermissible on rigid lenses because of the danger of cutting the eye.Contact lenses made of soft, tough, and resilient hydrogel materialcannot injure the eye, but there is no known manner of grinding orpolishing them to a feather edge.

An object of the invention, therefore, is the provision of a contactlens of soft polymeric material with an elasticity modulus up to 1,000kg./cm.2 whose outer and inner surfaces meet in an evenly sharp featheredge at an angle substantially smaller than 45. This primary object isto be achieved without impairing the optical performance of the lens,and without sacrificing the advantages of the afore-described method,such as the predictability of the refractive power of the lens producedand the absence of surface irregularities caused by contraction of thepolymerization mixture. Another object simultaneously to be achieved isthe absence of mechanical irritation of the cornea.

I have found that contact lenses consisting of hydrogels of the typedisclosed in the aforo-mentioned patent and having a feather edge may beprepared by polymerizing a suitable monomer composition in an open moldcavity having a concavely curved bottom surface and rotating about anupright axis transverse of that surface. There is obtained ashape-retaining polymer body having a con- Vex bottom surface conformingto the mold surface, and a concave top surface which has approximatelythe shape of a paraboloid formed by the rotation of the mold. Bycentrifugal force a change of curvature in the meniscus is caused. Ifthe diameter of the mold were large the top surface of the polymer bodywould have a precisely parabolic sectional shape. In a body Iwhosediameter is of the order of magnitude of a few millimeters, and whichweighs between 20 and 200 milligrams, as is inherent in a contact lens,the parabolic shape is distorted by the effect of surface tension whosemagnitude approaches the eiect of the centrifugal and gravitationalforces on the lens shape. The surface tension of a known polymerizationmixture, however, is constant and predictable at a fixed temperature,and the refractive power of a lens of the invention is thereforepredictable and reproducible. Lenses having powers coveringsubstantially more than the conventional range of -15 diopters to |15diopters are readily prepared by suitably adjusting the speed ofrotation of the mold during polymerization.

Most refraction defects of human eyes can be corrected by lenses havinga spherical outer surface, and prepared by polymerization of a suitablecomposition in a mold having a sperical concave surface. satisfactorilyoptical uniformity is achieved in a relatively small central portion ofthe lens. Since this surface portion is well known to be the only one ofimportance for correction of vision, such lenses are usually adequate.If perfect optical clearness of vision is required, a mold having aparabolic concave surface may be employed whereby homogeneity ofrefractive power may be obtained over a central lens portion whichextends Well beyond the largest practical iris opening.

Other features and many of the attendant advantages of this inventionwill be readily appreciated as the same becames better understood byreference to the following detailed description of preferred embodimentsthereof when considered in connection with the accompanying drawings inwhich:

FIG. 1 shows the disassembled elements of a spherical polymerizationmold of the invention in section on the axis of rotation;

FIG. 2 is a fragmentary perspective view of an apparatus formanufacturing contact lens blanks equipped with a multiplicity of moldsof the type shown in FIG. 1;

FIG. 3 shows a modi'ed mold and the lens blank produced therein inaxially sectional view;

lFIG. 4 shows yet another mold and the lens blank produced therein in anelevational View taken in a section corresponding to the line IV-IV ofFIG. 6;

FIG. 5 illustrates the device of FIG. 4 in an elevational sectioncorresponding to the line V-V in FIG. 6;

FIG. 6 shows the lens blank of FIGS. 4 and 5 in plan view; and

FIG. 7 shows an additional contact lens blank of the invention in a viewcorresponding to that of FIG. 6.

Referring now to the drawing, and initially to FIG. 1, there is seen adisassembled mold for producing less blanks of the invention. The moldconsists of a bottom part 1 having the shape of a low cylinder with ahemispherical, upwardly open cavity 21 centered on the cylinder axis.The mold bottom 1 tits snugly into an upwardly open recess 22 of a cupshaped carrier 2 coaxially mounted on a normally upright shaft 3. Thecarrier 2 is equipped with an integral, coaxial gear 6. When the moldbottom 1 is inserted in the carrier 2, its rim projects from the cavity22..

A cap or hollow cover 4 ts closely over the rim of the mold bottom 1.'Ihe carrier 2 and the cover 4 jointly form a substantially gas tightenvelope about the mold bottom 1 in the assembled condition of the mold.Access to the cavity 21 of the mold bottom is had through a conicallytapering aperture 23 in the cover 4. The aperture is normally sealed bya valve ball S under the weight of the ball. As shown in FIG. 1, acapillary tube 14 connected to a supply of inert gas such as argon ornitrogen free of oxygen is inserted in the aperture 23, displacing theself-closing valve ball 5.

FIG. 2 shows a multiple centrifugal casting or polymerization device ofa type preferred for manufacturing the contact lens blanks of theinvention. A supporting frame 24 is equipped with two spacedlysuperposed brackets 2S, 26. Vertically aligned openings in the bracketsprovide bearings for respective shafts 3 of a multiplicity of moldassemblies each composed of the elements shown in FIG. l. The spacing ofthe bearing openings in the brackets 25, 26 is such that the severalgears 6 of the carriers 2 meshingly engage so that all molds may berotated in unison at the same speed.

The shaft 3' of one of the mold assemblies carries a pulley 27 equippedwith a set screw 27 by means of which the pulley may be secured againstrotation on the shaft 3' in any axial position between the brackets 25,26. A belt 28 connects the pulley 27 to a drive pulley 8 composed of aplurality of lixedly connected, coaxial, grooved disks axiallyjuxtaposed on the output shaft 29 of an electric gear motor 7. The motoris mounted on the frame 24 by a pivot 30, and is spring-loaded in aconventional manner, not visible in the drawing, to keep the belt 28tight at all times and in engagement with any selected disk of thepulley 8.

A slide 31 is pivotally and axially movably mounted on a cylindrical rod32 of the frame 24. The rod 32 is parallel to the row of mold assemblieson the brackets 25, 26. The slide carries the delivery ends of threetubes 11, 12, 13, each capable of being inserted in the apertures 23 ofthe several covers 4 of the mold assemblies. The tubes 11, 12, 13 areconnected to proportionating pumps in a manner not further illustrated.The pumps are equipped with a conventional timer control (not shown) fordelivering precisely measured amounts of liquid chemicals in apredetermined sequence while the delivery ends of the tubes 11, 12, 13are sequentially introduced into the several mold assemblies. Theoperation of the control may be started by an operator.

Whether the drive shafts 3, 3 rotate at a desired uniform speed can beascertained by means of a disk 9 mounted on the shaft 3 and providedwith a row of uniformly spaced peripheral openings. A small neon lamp 33is arranged to illuminate the disk from below. |It is connected to asupply of alternating current in the usual manner (not shown). The disk9 and the lamp 33 thus constitute a convenient and precise stroboscopictachometer. The motor 7 is equipped with a built-in voltage regulator inorder to maintain the constant speed of the output shaft 29 necessaryfor precisely reproducible results of the centrifugal casting orpolymerization operation.

While only nine mold assemblies are shown in FIG. 2, the apparatusactually carries a row of fifteen assemblies, seven on each side of thedrive shaft 3. They rotate at the same speed which may be adjusted insteps of 0.5 revolution pery minute by shifting the axial positions ofthe pulley 27 and of the belt 28. Coarser speed change adjustments maybe made by changing the gears of the motor 7 in a conventional manner.

The temperature of the several molds is adequately controlled by anelectric heating rod 10 extending parallel to the row of mold assembliesand supplied with current through conventional controls such as avariable resistor (not shown). An operating temperature of 35 C. to 40C. is readily maintained in the mold cavities.

The following examples are illustrative of the operation of theabove-described apparatus.

EXAMPLE l The end of the tube 11 was inserted first in the cover 4 ofthe rst mold assembly shown at the left of FIG. 2, and the requiredamount of a monomer mixture was me tered through the narrow tube 11 intothe cavity 21 of the mold. The tube 11 was then replaced in sequence bythe tubes 12, 13, and the two components of a polymerization catalystsystem were supplied. The monomer mixture consisted of ethylene glycolmono methacrylate, 9.8% diethylene glycol mono methacrylate, and 0.2%ethylene glycol dimethacrylate which was the cross linking agent. Thetwo components of the catalyst system were an aqueous 5% ammoniumpersulfate solution and 2-dimethylaminoethyl acetate.

The mold cavity 21 had a diameter of 13 millimeters, and the totalamount of materials charged to the cavity Was 70 milligrams. The threeparts of the polymerization mixture were metered to the cavity at aratio of 72:2216,

whereby the overall initial composition of the polymerization mixturewas as follows (percent by weight):

u Ethylene glycol monomethacrylate 64.8 Diethylene glycolmonomethacrylate 7.056 Ethylene glycol dimethacrylate 0.144 Water 20.9Ammonium persulfate 1.1 2-dimethylaminoethyl acetate 6.0

The motor 7 was started before the feeding of the polymerizationingredients. After feeding, the capillary tube 14 was inserted in theaperture 23, and a stream of pure argon was admitted. It displaced alloxygen present within a few seconds while also stirring the mixture.When the tube 14 was withdrawn, the valve ball 5 dropped into its normalclosing position and sealed the mold cavity against atmospheric oxygenwhich would inhibit the polymerization in an uncontrolled manner.

While the mold rotated at a precisely uniform speed of 442 revolutionsper minute, the homogeneous liquid mixture in the first mold shown atthe left in FIG. 2 was permitted to polymerize for approximately 7 to 8minutes while the second third, and other molds were charged in sequencein the same manner as described hereinabove.

When the rotation of the first mold assembly was continued for 8minutes, the cap 4 was removed and the bottom 1 was upwardly withdrawn.lt contained a form-retaining, fully transparent, gel-like 'body in thecavity 21. The mold bottom 1 holding the body was replaced in theassembly by an empty bottom in the apparatus of FIG. 2 for furtherprocessing. The mold bottom holding the partly polymerized, sparinglycross-linked material was set aside for about 20 minutes untilcopolymerization of the lens blank was completed. The mold bottom wasthen immersed in distilled water at 85 C., whereby the blank swelled andcould thereafter be removed from the mold bottom without diiculty. Itwas then washed with distilled water until residual catalyst and othersoluble impurities were removed.

The blank was finally immersed for storage in physiological salinesolution until it reached osmotic equilibrium with the solution, andthereby with living body tissue, and assumed its final dimensions. Itsrefractive power was measured while the lens was immersed in the salinesolution, and the refractive power of the lens when in contact with airwas calculated as 4 diopters from the result of the measurements made.

The lens blanks formed in the other molds were processed in the samemanner, and precisely identical contact lenses were obtained. Itisevident that the apparatus shown in FIG. 2 may be operatedsemi-automatically with conventional controls to perform all operationsexcept the insertion of rnold bottoms 1 into the carriers 2, and removalof the bottoms from the carriers. Even these operations may bemechanized in an obvious manner.

Each polymerized lens was as clear as glass, of good optical quality,fully transparent, soft, and resilient. It had a very thin edge whichblended smoothly with the cornea surface when the lens was positioned onthe eye. Because of the absence of a sudden step at the boundary of thecornea and lens surfaces, the lens did not irritate the eyelids of thewearer, nor did the sharp edge irritate the cornea as would happen witha more rigid lens.

The lens could be inserted by holding it between the tips of threefingers and placing it on the cornea of the open eye. The lens clungfirmly to the eye surface, and centered itself on the cornea because ofthe almost perfect fit of the corresponding surfaces of lens and eye. Asmall difference between the curvatures of a cornea and of a lens of theinvention, as in cases of corneal astigmatism, is automaticallycompensated for by elastic deformation of the lens rim.

When the lens is stored in physiological saline solution prior to use,its water content is stable in contact with 6 living tissue, and itsdimensions therefore do not change while the lens is worn. Products ofmetabolism may freely diffuse from the eye to the free lens surface fromwhich they are removed by the wiping action of the eyelids, and by thetears. Atmospheric oxygen dissolves in the hydrogel of the lens, andpenetrates to the covered cornea. Properly chosen contact lenses of theinvention may be worn for long continuous periods without discomfort,and many patients do not even remove the contact lenses of the inventionduring the night.

The lens may be removed from the eye by gripping it with clean fingertips. It may then be stored in physiological saline solution to whichbacteriostatic or bactericidal agents may be added. Boric acid andantibiotics are representative of suitable agents.

EXAMPLE 2 A contact lens blank was prepared on the apparatus of FIG. 2in the manner of Example 1 from 90 milligrams of a modifiedpolymerization mixture in a hemi-spherical mold of 10 millimeterdiameter. The mixture had the -following initial composition (percent byweight):

The mold was rotated during polymerization at 370 revolutions perminute. The finished lens had a refractive power of -7 diopters.

EXAMPLE 3 The apparatus of FIG. 2 was operated at 350 revolutions perminute, and a spherical mold having a diameter of 17 millimeters wascharged with 50 milligrams of a polymerization mixture having thefollowing composition (percent by weight):

Ethylene glycol monomethacrylate 60 Diethylene glycol monomethacrylate17.7

N,Nmethylenebis-methacrylamide 0.3 Ammonium persulfate 1.0 p-Toluenelsulinic acid 1.0 Water 20 The ammonium persulfate was introduced intothe mold 1n the form of a 5% aqueous solution as described above. Cupricchloride was admixed to the persulfate solution at the Vrate of one dropof a 0.1% aqueous solution of CuCl2-2H2'O per 20 milliliters of thepersulfate solution.

A temperature of 40 C. was maintained while the mixture was rotated inthe mold for 12 minutes. The mold was then removed from the apparatus,and its contents were permitted to mature for 30 minutes. The procedureof Example 1 was followed in all other respects. The lens ultlmatelyobtained had a refractive power of 0 diopter.

EXAMPLE 4 Ethylene glycol monoacrylate 63 Diethylene glycol monoacrylate11.6

Diethylene glycol dimethacrylate 0.4 Potassium persulfate 1.2 Water 20Bis- (p-toluenesulfomethyl) -methylarnine 3 8 7 The last mentionedcompound was prepared in a known manner from p-toluenesulfochloride,formaldehyde, and

had the formula CHs-CHi-SOz-CHz N-CH;

CI-Ig-CeHi-SOg-C 2 The refractive power of the lens prepared from theblank in accordance with the general procedure of Example 1 was +2ldiopters.

EXAMPLE A hemispherical mold having a diameter of millimeters waschanged with 150 milligrams of a polymerization mixture of the followingcomposition:

Ethylene glycol monomethacrylate 80 Diethylene glycol monomethacrylate 8Glycerine trimethacrylate 0.4 Ethyl-azo-bis-isobutyrate 2 Water 9.6

The mold contents were rotated at 375 revolutions per minute in themanner described in Example 1 while they were covered with a cover 4made from transparent polymethyl methacrylate, and exposed to theradiation of a mercury -vapor lamp placed 150 millimeters from the mold.The lamp was provided with a lter selectively permeable to radiation inthe range of wavelengths between 3200 and 3700 angstrom units. Heatingof the polymerization mixture by mercury radiation outside the desiredultraviolet range was thus avoided. Copolymerization was complete after30 minutes rotation. The lens blank was immersed in 50 percent aqueousethanol to swell it, and was otherwise handled as described inExample 1. The refractive power of the finished lense was +1 diopter.

The shape of a lens blank may be controlled not only by the size andshape of the mold, by the amount and nature of the polymerizationcharge, and by the rotary speed of the mold and of the charged uringpolymerization, but also by the relative position of the axis ofrotation of the axis of rotation, of the center or axis of the mold, andof a vertical line drawn through the mold center.

EXAMPLE 6 The procedure of Example 1 Was repeated, but modified bytilting the axis of rotation of the mold 30 with respect to a verticalline by the use of a mold having a radius of central curvature of 7.5millimeters, and by a speed of rotation of 400 revolutions per minute.The lens ultimately obtained had a spherical refractive power of -5diopters.

When the axis of rotation is inclined relative to the direction ofgravitation, or when the axis of rotation does not pass -through theoptical axis of the lens, a prism component is added to the sphericalcomponent of lens refraction. A hydro'gel lens of the invention havingboth spherical and prismatic refraction components is useful forimproving the vision of an aphakic eye, as after a one-side cataractoperation. The prismatic lens component shifts the lield of vision ofthe lens-less eye to a more normal position relative to the field visionof the other eye.

Modied lens shapes that can be obtained by the choice of suitablycontoured mold bottoms, by tilting the axis of rotation of the apparatusof FIG. 2, and by mounting mold bottoms eccentrically with respect tothe axis of rotation are illustrated in FIGS. 3 to 7.

FIG. 3 is an elevational axial section through a mold bottom 15 and thelens blank 16 formed therein by polymerization while the mold rotates.The mold bottom 16 has a central surface portion 17 and a peripheralannular surface portion 18. Both portions 17, 18 are of spherical shape,but the radius of curvature of the peripheral portion 18 is smaller thanthat of the central portion 17. Such a mold is advantageous in theforming of lenses having a low refractive power and small diameters inthat it provides for a lens thick enough for mechanical strength, yetretaining the feature of inner and outer lens surfaces meeting at a verysmall acute angle in a feather edge. The lens illustrated in FIG. 3 hasa refractive power of less than '-1 diopter, yet the concave and convexedge surfaces meet at an angle substantially smaller than thirtydegrees.

FIGS. 4 to 6 illustrate the making of a lens blank 19 which has only oneplane of symmetry as compared to the infinite number of planes ofsymmetry in a lens blank produced by rotation of a spherical mold abouta vertical axis passing through the center of curvature of the mold. Thelens blank 19 and the mold bottom 20 in which it was made areillustrated in FIG. 4 in section of the single plane of symmetry. FIG. 5shows the mold bottom and lens in section on a plane perpendicular tothe plane of symmetry through the optical axis of the lens, and FIG. 6shows the lens blank formed as viewed in a direction parallel to theplanes of FIGS. 4 and 5.

The molding surface of the mold bottom 20 is of elliptical section inall planes parallel to the outer radial faces of the mold bottom. Thecontours of the molding surface on the levels L1, L2, L3, and L4 inFIGS. 4 and 5 are indicated by broken lines C1, C2, C3, and C4 in FIG.6. The mold was rotated during polymerization of the blank about an axisoffset by an angle of 5 from the center line of the mold in the plane ofsymmetry. The contact lens blank 19 has a spherical refractive power of+16 diopters and a superimposed prismatic refractive power which makesit suitable for use on an aphakic eye. Its elongated shape facilitatesautomatic centering of the contact lens over the iris as the eyelid ismoved over the lens.

The lens blank 34 illustrated in FIG. 7 together with the contour linesC'1 to C4 of the corresponding mold bottom, not itself shown, has noplane of symmetry. As is evident from the contour lines, lens blanks 34was polymerized in a rotating mold having no plane of symmetry androtated about an axis outside the centre. The lens blank 34 has aprojecting lug 35 which extends into the inner corner of the eye in theoperative position of the lens, thereby ensuring precise alignment ofthe lens with the pupil as is necessary in cases of severe astigmaticrefraction defects. The lug 35 also provides reliable contact of thelens with a source of aqueous liquid for maintaining the lens in theproper swelled condition.

The normal automatic aligning effect of the moving eyelid on a contactlens of the invention placed over the cornea is enhanced by shallowcircumferential grooves or depressions in the peripheral portions of thelens which suiciently engage the moving eyelid yet do not causeirritation of the same. They are placed outside the field of sight, andthus do not interfere with vision.

Those skilled in the art will readily provide suitably contoured moldsand rotate them about eccentric axes or axes obliquely inclined relativeto a vertical direction in order to produce lens 'blanks of theinvention which meet all known requirements as to refractive properties.They may be further guided by the following empirical formula forderiving the spherical refractive power D of a contact lens of theinvention (in diopters) from the parameters of the molding apparatus andthe polymerization mixtures:

N--i s 4 s n n RB naam/tI @es a 1.218 r1/U GOS a is the surface tensionthereof in dyn/cm.;

p is the angle defined by the axis of rotation and a vertical line; and

n is the speed of rotation of the mold in revolutions per minute.

The formula reflects the fact that the refractive power of a lens of theinvention is strongly influenced by the surface tension of the fluidpolymerization mixture within the normal range of R0, namelyapproximately to 7 millimeters.

Regardless of the properties of the mold or of the nature of thepolymeric hydrogel material which constitutes the nished lens, thelenses of the invention have sharp edges defining angles smaller than 45degrees in any plane including the optical axis of the lens. The edgeportion is thin and smooth, and closely adheres to the surface of theeye. The edge of the lens is as invisible to an onlooker as the mainbody of the lens.

In view of the entirely physical and mechanical nature of the parameterswhich enter into the above equation, the chemical nature of the lensmaterial may be varied at will as long as the product obtained bypolymerization of a uid mixture including water-soluble monomers andsubsequent swelling of the blank in an aqueous liquid is transparent,physiologically tolerated, soft, and dimensionally stable in contactwith body fluids. The process of making the lens blanks of the inventionis performed most readily with polymerization mixtures Whose rate ofpolymerization is high at ordinary room temperature (about l5 to 25 C.)or at temperatures only slightly above room temperature (25 to 40 C.).The term polymerization as employed in this specification will beunderstood to include not only the chain polymerization of unsaturatedmonomers, but also the polycondensation and polymerization of cyclicmonomers (Cpolym erization, W. H. Carothers), polyaddition reactionssuch as those between polyisocyanates and compounds having reactivehydrogen atoms, and similar well-known reactions which producepolymeric, sparingly cross-linked bodies capable of swelling in aqueousliquids.

Any by-products formed during polymerization must be water soluble ifthey would interfere with proper use of the contact lens formed. In thepreparation of polymeric hydrogels from such highly reactive monomers asthe acyl halides derived from polybasic carboxylic acids, glycolates ofalkali metals are therefore preferred reaction partners in the formationof polyesters since the alkali metal halides formed as by-products mayreadily be washed out of the hydrogel by water.

In order to produce a fully transparent lens, the polymerization mixtureshould constitute a single phase throughout the polymerization period,that is, miscibility of the components of the mixture should bemaintained while polymerization proceeds. Water is not an indispensablecomponent of the mixture, but may be replaced in part or entirely byorganic solvents readily soluble in water, such as the water solublelower aliphatic alcohols, also polyvalent alcohols such as glycols andglycerol, dioxane, and the like, which are displaced by water when thelens blank is immersed in Water for a suflicient time. When an anhydrouspolymerization medium such as dioxane is used, ionic polymerizationcatalysts such as an alkoxy lithium compound may be employed.

Extensive clinical tests have shown that the glycol and glycerol estersof acrylic acid and loWer-alkyl-acrylic ester form copolymers of therequired optical and mechanical properties which are well tolerated bythe human body in periods of contact which extend over serveral years.Preliminary tests, however, indicate that suitable hydrogels may beprepared from other hydrophilic monoolenic monomers mixed 4with smallamounts of diolenic cross linking agents.

Such mono-olefnic compounds include, for example, themonoglycol-monoallyl ester of malic acid, the diglycol `monoallyl esterof citric acid, the diglycol ester of itaconic acid, and diglycolfumarate as the minor component of a mixture with a major amount ofglycol monomethacrylate. Diolenic or polyoleiinic cross-linking agentsfurther representative of those useful for the purpose of the inventioninclude diallyl malate, diallyl-monoglycol citrate, allylvinyl malate,glycol vinyl allyl citrate, monoglycol monoallyl itaconate, monoglycolmonoallyl fumarate, and polyesters prepared by condensation of maleicanhydride with triethylene glycol and having a sufliciently lowmolecular weight to be water soluble.

Implantation tests in rabbits indicate that lenses made from the neutralnon-ionogenic hydrogels produced from these monoolenic and polyolenicmonomers are well tolerated even when the implant replaces the naturallens within the eye. Ionogenic hydrogels, however, are also adequatelytolerated for use in contact lenses when the ions formed do notsignificantly affect the hydrogen ion concentration of the tissue withwhich they are in contact.

It is therefore possible to replace the dimethylaminoethyl acetate andother activators described in Examples l to 6 by 5 percent of thehydrochloride of dimethylaminoethyl methacrylate. The latter compoundnot only increases the rate of polymerization, but it is alsoincorporated in the macromolecule produced. Contact lenses consisting ofhydrogels so modified and later neutralized do not noticeably irritatethe eye on which they are Worn. Similarly, the presence of 2 percentfree methacrylic acid in the monomer mixture is not objectionable. ifany free carboxyl groups present in the hydrogel of the finished lensare neutralized by simultaneously present alkali, for example, by sodiumions supplied by immersion of the lens blank in a Weekly alkalinemedium.

The monomeric esters of acrylic and lower-alkyl-acrylic acids with.polyhydric alcohols as illustrated in the examples are readily availabein a purity sufficient to give the desired optical properties to thehydrogel lens ultimately obtained. They are stable at low temperaturefor long periods Without the need for polymerization inhibitors. Thesparingly cross-linked hydrogels made from these monomers are chemicallyand dimensionally stable 'while stored in physiological saline solutionand under the conditions of use as contact lenses. They may besterilized in boiling Water. They are capable of absorbing considerableamounts of water-soluble medicinal compounds, such as oxytetracyclinehydrochloride or boric acid, from the aqueous solutions of thesecompounds, and to release them to an eye on which a hydrogel lens isplaced. Conjunctivitis and other ophthalmic diseases may thus be treatedby the use of the contact lenses of the invention.

The catalyst system employed for initiating polymerization of themonomer mixture may be chosen from a wide variety of well-knowncompounds and compound combinations as partly illustrated by thespeci'lc examples. Catalyst systems which provide free radicals at thedesirable process temperatures of 20 to 50 C. are preferred. Redoxsystems have been found very convenient. Their oxidizing component maybe hydrogen peroxide, the peroxide derivative of an acid, or a per-acidsuch as persulfuric acid andits salts. The reducing component may beferrous sulfate when hydrogen peroxide is the oxidizing component, andwill also act as a polymerization accelerator, as is well known.

Other oxidizing components for redox catalyst system include the Watersoluble salts of peracetic and perboric acid, cumene hydroperoxide, andperacetals of aliphatic and alicyclic ketones. It is a common feature ofthese compounds that they are soluble in the polymerization mixture ineffective amounts, and that they and their decomposition products can beremoved from the lens blank or the nished lens by washing with Water.Reducing components for a redox catalyst system also may includesultites, pyrosulftes, hydrosulfites, thiosulfates of the alkali metalsand of ammonium, formaldehyde bisulte, ascorbic acid, glucose, and manyother compounds 11 which will readily suggest themselves to thoseskilled in this art.

Metals forming compounds in different valence states may be used intrace amounts as valuable addition agents to the catalyst system. Theuse of copper ions has been illustrated in Example 3, but iron, cobalt,nickel, manganese, cerium, and silver are similarly effective. The rangeof polymerization initiators and accelerators suitable in thepolymerization process of the invention further includes azo compoundssuch as the esters and the nitrile of azo-bis-isobutyric acid. Azocompounds are employed in such amounts that the gaseous nitrogen formedby their decomposition dissolves in the polymerization mixture, and isgradually released by diffusion without forming visible bubbles.Polymerization may further be initiated by electromagnetic radiation ofshort wavelength, such as the ultraviolet rays described in Example 5.Radiation sources may be employed in a known manner in conjunction withcatalyst systems consisting of ferrie chloride and an organic acid, ofperchlorates or potassium permanganate and oxalic acid or benzoine.

The ratio of the components in the polymerization mixture may be variedwithin fairly wide limits. Some empirical rules have been found usefulin producing consistently acceptable lens blanks. The solvent content ofthe polymerization mixture should be between and 50%, preferably between15% and 40% by weight, the solvent being either water or a `watersoluble organic liquid. When these limits are observed, a clear,homogeneous blank is produced by polymerization, and is capable ofswelling substantially when subsequently brought into contact withwater. The desired mechanical properties of the lens are obtained whenthe cross-linking agent amounts to only a small fraction of the totalamount of monomers present, and to only 0.1 and 1.9 percent of thepolymerization mixture by weight.

The apparatus illustrated in FIG. 2 may be modified in many aspectswithout departing from the spirit of the invention. lOne of threedelivery tubes 11, 12, and 13 and the corresponding pump need not beused if a redox catalyst system is employed. The oxidizing component ofthe system may be pre-mixed with a portion of the monomers and/or thesolvent, and the reducing component may be pre-mixed with the remainderof the monomers and the solvent. The pre-mixes are relatively stable,and precise metering of the very small amounts of catalysts isfacilitated by the dilution with the very much larger amounts ofmonomers and/or solvent. The cross-linking agent is preferably added tothat premix which is less prone to polymerize spontaneously.

The materials of construction employed for the apparatus of FIG. 2 arenon-critical with the exception of the mold bottom. Many materials areavailable for making a mold bottom whose molding face can be accuratelyshaped and very smoothly finished. Glass and silica may be shaped to thedesired contours at elevated temperature and are chemically inert to thepolymerization mixture. Chromium plated metallic mold bottoms also havebeen found acceptable. Useful molds have been prepared from pure,unfilled phenolic, carbamide, or -melamine resins by compression moldingat elevated temperature with a highly polished metal die. Good silicateglass, however, olers the preferred combination of chemical inertness,smooth surface, ready formability, and low cost.

Although hydrogels having in swollen state a very low elasticity`modulus of e.g. 0.1- kg./cm.2 are preferred materials for contactlenses according to the present invention, it is also possible to useinternally plasticized soft, rubbery hydrophobic polymers and copolymerssuch as copolymers of butyl methacrylate with vinyl acetate, styrene,vinyl acetals etc., advantageously with admixture of less than 2% of across-linking agent such as divinyl benzene. Copolymers of vinyl acetalsmay be prepared in known manner by saponifying the vinyl acetatecomponent and treating the copolymer with aldehydes. Such softcopolymers having E-modulus lower than 1,000 are known. They were usedhitherto e.g. as thermoplastic foils for binding or cementing wood,glass and other materials.'

It should be understood, of course, that the foregoing disclosurerelates only to preferred embodiments of the invention, and that it isintended to cover all changes and modifications of the examples of theinvention herein chosen for the purpose of the disclosure which do notconstitute departures from the spirit and scope of the invention setforth in the appended claims.

What is claimed is:

1. A method of preparing a corneal contact lens comprising the steps ofproviding a mold of an overall diameter less than 17 mm. and having aconcave continuously curved upwardly extending solid supporting surfaceof an extent greater than the lens to be formed therein, introducingonto said supporting surface a fluid polymerization mixture essentiallyconsisting of a homogeneous liquid solution comprising a major amount ofa monomer consisting of a water soluble =monoester of acrylic ormethacrylic acid with a glycol, a minor amount of cross linking agentconsisting of a diester of acrylic or methacrylic acid with a glycol, apolymerization catalyst and a solvent selected from the group of waterand water soluble organic solvents selected from the lower aliphaticalcohols, glycol, glycerine and dioxane; said liquid solution having asurface in contact with said mold and a free surface directed awaytherefrom, rotating said mold about an axis transverse to the supportingsurface at a speed in excess of 300 r.p.m. sufficient to cause radiallyoutward displacement of said fluid mixture under centrifugal force for atime sufficient for polymerization of said monomer with saidcross-linking agent, selecting said iiuid mixture and said mold andmaintaining the polymerizing conditions thereof during rotation tothereby form a shape retaining body wherein the convex surface is formedby the corresponding concave surface of the mold and the concave surfaceis formed having a shape different from a paraboloid the two 'surfacesterminating in a tapered periphery having a thin feathered edge.

2. The method according to claim 1 including the step of tilting theaxis of rotation at an angle to a vertical axis during rotation of saidmold.

3. A method as set forth in claim 1, wherein said mixture has a Weightof between 20 to 200 milligrams, and said speed of rotation is between300 and 500 revolutions per minute.

4. A method as set forth in claim 1, wherein said supporting surface isspherical.

5. A method as set forth in claim 1, wherein said supporting surface isparabolic.

6. A method as set forth in claim 1, wherein said solrvent constitutes'between 5 percent and 50 percent of said mixture.

7. A method as set forth in claim 1, wherein said free surface iscovered by an inert gas free from oxygen during said rotation.

8. A method as set forth in claim 1, wherein the amount of saidcross-linking agent is 0.1 to 1.9 percent by weight of said mixture.

References Cited UNITED STATES PATENTS 2,241,415 5/'1941 Moulton 264-412,280,636 4/ 1942 Kraft 2-64--31'1 2,972,782 2/ 1961 Archibald 264-11'2,976,576 3/1961 NVichterle et al. 264-1 3,010,153 ll 1/:1961 lBittner26-4-l 2,510,438 l6/ 1950 .T ouhy '351-160 2,544,246 3/1951 Butterfield351-1'60 3,227,507 fl/ 1966 Peinbloom 264-1 3,408,429 10/ 1968 Wichterle264-1 (Other references on following page) 14 FOREIGN PATENTS Sears etal., College Physics, 1960, Addison-Wesley 814,009 s/1959 .Great Britain264-1 Pub-Q0 LDHdOBPP- 260464-,

Great Britain Obllg et al., Contact Lens; pp. 21, 22, 104, 921,4161x2/1954 1Germany ssl-160 105- 5 JULIUS FROM'E, Primary Examiner A. H.KOECKERT, Assistant IExaminer Modern Plastlcs, August 1957, pp. L16,117.

Weber et al., Physics 1957, lMcGraw-Hill, pp. U'S' CL X'R' 180-181. 10260-80.73, 80.75; 264-311; S51-160, 177

OTHER REFERENCES

